This invention relates to methods and apparatuses for measuring the average velocity of an open channel flow using electromagnetic radiation such as, for example, laser signals to determine velocity by Doppler shift techniques without physically contacting the flow but measuring the flow below the surface.
In one class of Doppler shift remote sensing of the velocity of an open channel flow, a laser beam is transmitted to a flow from above and the backscatter received from scatterers in the fluid such as bubbles, solid objects (e.g. debris) or microscopic particles (e.g. colloid) causing turbidity, carried by the flow are sensed. The Doppler shift in frequency between the transmitted signal and the returned signal is used to determine the velocity of the portion of the flow sampled by the laser beam. In one embodiment, the average volumetric flow rate of the sample is determined by combining the average velocity of the flow stream measured by the laser Doppler velocimeter with other data such as the height of the flow in the channel and the geometry of the channel. In another embodiment, the average flow velocity is determined by measuring the velocity of the flow at multiple locations across the flow and combining the readings to arrive at an average.
In one prior art Doppler-shift flow meter in this class, a laser transmits a signal to the surface of a flowing stream where objects on the surface reflect signals back. The Doppler shift between the transmitted and reflected light is used to determine the velocity of the surface of the flow. The localized mean velocity is calculated from the surface velocity while the average velocity of the entire flow is calculated from the mean velocity using the level of the flow and the cross section of the stream bed. A system of this type is disclosed in U.S. Pat. No. 5,811,688. This technique has the disadvantages of being inaccurate under some circumstances due to the difficulty in accurately arriving at the mean flow velocity from the surface velocity, and of detecting a signal when there are few suitable reflectors on the surface.
In still another velocity measuring, Doppler-shift, prior art technique, frequency modulated laser beams are transmitted to a target from a laser diode and the velocity of the target is determined from the Doppler shift of harmonic frequency reflected signals and the transmitted signals. This technique is disclosed in U.S. Pat. Nos. 6,885,438 and 7,061,592. This prior art is taught only in connection with solid targets with a focal point on the surface of the target and thus does not relate to some of the unique problems associated with measuring open channel flows.
In still another prior art type of fluid velocity measuring technique, self-mixing and self amplifying laser diodes transmit beams to two spaced apart focal points within the flowing stream. Flow velocity is measured by the time it takes for fluid to move between the two points. This technique relies on the identification of unique signatures within the flow. The technique is described in “Low Cost Velocity Sensor Based on the Self-Mixing Effect in a Laser Diode”, Opto-Electronics Review 11(4), 313-319 (2003) and in “A Simple L2F Velocimeter Based on Self-Mixing of Laser Diodes”, 14th Int Symp on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 0710 July, 2008. While these methods do not use Doppler shift, one of them mentions that self-mixing diode lasers may be used in Doppler shift velocimeters.
In still another fluid velocity measuring Doppler-shift prior art technique, two laser beams are caused to intersect at a point in the flowing stream and the velocity at that point is determined by the Doppler shift of the scattered light. This technique is disclosed in U.S. Pat. No. 4,026,655. This patent describes the use of this technique in measuring air speed and does not apply it to measuring velocity in an open channel flow carrying reflecting objects.